these two words escape me.

The first means: of unknown origin such as an ancient manuscript.

The second means: authorship credited after death

Posthumous is used in this context, but it's a more general term than just one referring to credit of authorship.

posthumous! thank you.

the other is still haunting me. anyone?

Anonymous probably isn't what you are looking for if you mean the origin rather than author is unknown. Is is a single word or would unknown provenance work?

Perhaps for the first, apocryphal would do?

cryptogenic has exactly that sense, but is usu. used of diseases.
anonymous, as was suggested, can take that sense.

Unidentified, Obscure. Not so sure about the latter.

Welcome, wutword. All I can recall ever seeing in, say, a dictionary is "origin unknown", but. Here's a link to a thesaurus response to my putting in the word "unknown":
Roget
Possibly one of these will be what you're looking for.

APOCRYPHAL!!!!!!!!

thank you! That is the word!

My buttocks have seazed to itch.

I am not sure whether that is a strange reaction to vocabulary expansion or if it should be a new thread under "Information and Announcement"
Either way, congratulations.

that sounds painful.

I'm happy to have helped you out! :0) I hate it when my buttocks itch, too... ;0)